Satellite telecommunication systems have been applied to a variety of commercial purposes over the past thirty years. These satellites first started out as low-power (e.g., several hundred watt) devices that required very powerful satellite Earth stations having antenna dishes on the order of 30 meters in diameter. A typical application in the earlier days of satellite communications was simple point-to-point trunking for public telephone companies.
Technology has progressed to the level where modern satellites are now highly sensitive devices which are capable of providing high-power transmissions. By way of example, today's satellites have power resources in the range of approximately 10-15 kilowatts. The development of more advanced satellites has lead to corresponding reductions in the size of Earth stations.
Advancements in satellite communication system technology have lead to the development of three basic categories, as defined by the International Telecommunications Union (ITU), of satellite services: fixed, mobile, and broadcast. Fixed systems, as the name implies, employ fixed or stationary terminals. Fixed systems have traditionally been utilized for point-to-point telecommunications trunking as part of public telephone networks, or for large private telecommunications networks. Since these types of communications tend to be reciprocal in nature, the same frequency and the same size terminal is normally used for both transmission and receiving of signals. Conventional fixed systems, therefore, typically utilize a single set of transmission channels (i.e., transponders). By way of example, U.S. Pat. No. 5,163,160 discloses a satellite transponder in which the transmitter is tuned to the same frequency as the local oscillator of the receiver. A frequency control circuit having a local oscillator applicable to the receiver of a satellite telecommunications apparatus is also described in U.S. Pat. No. 5,479,136.
Modern fixed satellite systems that provide two-way communications tend to be relatively expensive, highly application-specific systems. Due to these limitations, fixed, two-way satellite systems have generally been restricted to use by large corporations to provide data transmission, or by public telephone companies for basic trunking.
In mobile satellite systems, the user terminal can move from one location to another. An example of a mobile satellite system is the International Maritime Global Satellite (INMARSAT).TM. system. Planned mobile cellular telephone satellite systems include Asia Cellular Satellite (ACeS).TM., Iridium.TM., Globalstar.TM. and ICO Global.TM.. Details of a mobile cellular telephone satellite system are disclosed in U.S. Pat. Nos. 5,433,726 and 5,439,190. A telecommunications network applicable to mobile terminals which employs a constellation of orbiting satellites placed in low-altitude polar inclined orbits is described in U.S. Pat. No. 5,471,641. U.S. Pat. No. 5,485,170 discloses a terminal suitable for use in a mobile telecommunication system.
A major problem associated with mobile systems is the limitation on capacity. Mobile systems are limited as to the number of users that they can accommodate, largely because the available bandwidth is restricted by international regulations.
Broadcast satellite systems are in service today across many parts of Europe, North America and Asia. They typically provide television broadcast services to receive-only subscriber terminals which employ small antennas limited in use for signal reception. Although broadcast systems are becoming ubiquitous throughout many regions of the world, they suffer the drawback of only providing one-way telecommunication service. In other words, the subscriber terminals are incapable of transmitting signals back to the broadcasting satellite. This means that any requests or other signaling initiated by the end-users must take place through ordinary terrestrial telecommunication lines.
Thus, there is a strong need for a satellite-based telecommunications system that can provide increased and improved services--well beyond the capabilities of present fixed, mobile, and broadcast satellite systems--to serve the mass public market. Instead of simple one-way broadcasting, the general public has a need for two-way multi-media services.
As will be seen, the present invention is a satellite telecommunications system that provides a great variety of services--including telephony, interactive television, video on-demand, Internet access, data communications, etc.--available on-demand, directly to the end user. The invented system combines low-cost user terminals with high system capacity, making the telecommunications system highly attractive to mass market usage. Moreover, all of these advantages are achieved without the need to rely upon well developed terrestrial telecommunications infrastructure.